As a renewable clean energy, ethanol has become a research focus in the field of industrial biotechnology. Free cells are used in traditional ethanol fermentation technology, in which the yeasts flow away with the fermenting mash, resulting in in-sufficient yeast concentration in the fermentation tank, slow ethanol fermentation and long fermentation time.
In recent years, immobilized yeast has obtained more and more attention and application in the fermentation production of fuel ethanol. The nature of the immobilized cell technology is to use physical or chemical means to localize free cells in defined space region but make them keep catalytic activity and reused repeatedly so as to reduce cell loss due . With the development of the immobilization technology, studies on immobilization method and carrier material become more and more extensive. It is found that immobilized yeast fermentation can affect physiological properties and metabolic activity of yeast cells, such as cell morphology, intracellular osmotic pressure and membrane permeability and so forth. Meanwhile, immobilized yeast can also improve the tolerance of yeast cells to fermentation inhibitors and fermentation environment.
Due to gathering of massive yeast cells on carriers after immobilization, certain concentration advantage is obtained which protects the yeasts from being contaminated by other microbes.
There are more and more immobilization methods applied to yeast cells at present and common immobilization methods include embedding method and adsorption method.
The embedding method is the most common and the most extensively studied immobilization method, which is a method to immobilize microbial cells by intercepting the cells in the pores of a gridding made of water-insoluble polymer compounds. The embedding method generally includes the embedding method using natural carriers and the embedding method using synthetic organic polymer carriers. The natural embedding carriers include agar, carageenan, calcium alginate, sodium alginate and polyvinyl alcohol and the like. They have advantages in non-toxicity to organisms, easy molding and high immobilization density, but they have low mechanical strength, weak mass transfer and poor resistance to microbial decomposition; while the synthetic organic polymer carriers such as polyacrylamide, polyvinyl alcohol and polyacrylic acid have high mechanical strength and stable chemical properties, but forming a polymer network during immobilization requires harsh conditions which would cause great harm to cells; on the other hand, since cells are inside the network materials, there are some problems of weak mass transfer, thalli death at the later stage and low catalytic efficiency.
The adsorption method utilizes the adsorbability of microorganism to a solid surface or the surface of other cells to make it adsorbed to the surface of water-insoluble carriers and thereby immobilized. The adsorption method includes physical adsorption and ion adsorption. The physical adsorption immobilizes a microorganism by using the materials with strong adsorbability, such as silica gel and active carbon; while the ion adsorption makes microbial cells adsorbed to ion-exchanger by electrostatic interaction. The advantages of the adsorption method lie in easy operation, utilizing ion-exchanger which is stable and not easy to be decomposed, and less effect on cell activity, but the shortage is that the adsorbent material in the method cannot be used repeatedly very well.